298 FLIGHT. MARCH 21, x935.
whether a height above 8,000-10,000ft.
will ever be commonly used unless air
tight cabins are introduced.
So much for the aspects of high-
altitude flying which especially affect
the occupants. When we turn to the
technical side, some advantages and a
good many difficulties are found.
It is a fundamental law of aero
dynamics that the drag of an aeroplane
varies as the square of the speed and
inversely as the air density. On the face
of it, therefore, one would say that, by
going up to great heights, the drag of
the aeroplane would be reduced, so that
for the same power one could travel very
much faster.
(Above) In this Farman monoplane,
specially built for high-altitude attempts!
the pilot flew for a time with his head
outside the cockpit ; then, when sufficient
height was gained to make risk of
collision with other aircraft improbable,
he lowered himself in the cockpit and
shut an airtight door above his head.
(Left) The Vickers " Vespa " biplane, with
special supercharged Bristol "Pegasus"
engine, on which Mr. C. F. Uwins gained
the altitude record for Britain in 1932
with a height of 44,000 ft. The record
is now held by a " Pegasus "-engined
Caproni biplane with a figure of 47,353 ft.
ft!*;
Unfortunately, there are many factors which make the
attainment of these higher speeds difficult. The first one
is related to the engine. A petrol engine, as everybody
knows, draws into its cylinders a certain mixture of air
and petrol, the oxygen in the air enabling combustion to
take place. Owing to the decrease in air density at
heights, the weight of air drawn in on each induction
stroke of the piston is less than the weight drawn in at
ground level, although the volume is the same. To obtain
a proper gas mixture it is, therefore, necessary to reduce
the quantity of petrol, otherwise the mixture will be too
rich. The cutting down of the amount of petrol admitted
results at once in a fall in engine power, this fall being
approximately proportional to the altitude. At 20,000ft.,
for example, where the density is roughly one-half the
ground level density, the engine power will also be reduced
to approximately one-half, and one's hopes of attaining
the extra speed are doomed to failure unless means can be
found for supplying extra oxygen to the engine.
Such means have been found in the supercharger, which
is, basically, merely a centrifugal blower, driven by the
engine and causing a raised pressure in the induction pipes
of the engine. That, one would say, has solved the
problem. But, unfortunately, it has brought with it some
other difficulties. If the engine was originally designed
for giving its full power at ground level when naturally
aspirated, it will receive too large a charge of mixture
when the supercharger is brought into action at a low
height, and the engine may not be able to stand the extra
strain. During the earlier stages of the climb, it may be
necessary to put the supercharger out of action until a
certain height has been reached. Then, when the super
charger is brought into use it will enable the engine
to maintain its ground level power up to a height which
depends upon the degree of supercharging. This height
may be 5,000ft. or 15,000ft., according to the design.
Whatever the altitude is, the effect is to make that height
the equivalent of ground level as far as the engine is
concerned, the power beginning to fall off from that altitude
onwards, just as it previously did in the unsupercharged
engine from ground level onwards.
It will be obvious that power is absorbed in driving the
supercharger, so that here is an obvious limit to the gam
that can be expected. For reaching heights much above
40,000ft. it is no longer practicable to get the necessary
boost with one supercharger, and two or more in senes
must be used. This again means a considerable dram oil
the engine's power, and steps have to be taken to cool
the air during its passage from one supercharger to the
When heights such as those contemplated are involved,
the airscrew problems make themselves felt. An ordinary
airscrew with fixed blades has its maximum efficiency^
some particular value of forward speed, rotational spe
and diameter. If the airscrew is designed to give g«w
efficiency near the ground, at the speeds attained ere,
its efficiency will be poor at a great height and at a m^
increased forward speed of the aircraft. On th*L
hand, if the airscrew is designed for the altitude COT an ^
its efficiency at low altitudes will be poor, and the
will suffer badly. $
Two remedies are obviously possible. Either ^
introduce a gear box, or one can design the air ^
that the pitch angle of its blades can be varied. ^^
alternative is the one which is coming int°ttebfin»
use, a number of controllable-pitch airscrew UBC, <X llUlliUCl UX <_<_I1IC11J11U.LI1G-JJ'JI>-1J ,,
available. Just as the supercharger restorfallabie.>:i>
of the engine at great heights, so the controiia
airscrew restores the airscrew efficiency. follows
From the fundamental laws of aerodynamics ^
lat. "other thines being equal" as the matn ]v that uiaL, other things being equm »*> —- inverse
have it, the speed of an aeroplane sn0uldr]!ln^efs to say,at That is
as the square root of the air density, inar • ^^
20,OOOft. the speed should be increased by aD ^
cent, and at 40,000ft. it should be nearly don only
hied.
beca"se
practice this is not, of course, possible, ""'"'['jisof*'
of the power absorbed in driving the engine ^
number of other reasons. A very substan